1. Field of the Invention
The present invention relates to an image processing apparatus for identifying fingerprints and palmprints, in particular, an image processing apparatus for acquiring ridge line images from skin pattern images.
2. Description of the Related Art
As a method for extracting a ridge line image from a skin pattern image, a technology disclosed as for example Japanese Patent Examined Publication No. 5-746 titled "Fingerprint Image Coding Method" is known. In the fingerprint image coding method, a fingerprint image is divided into small regions. For each small region, two-dimensional Fourier transformation is performed. With information of each transformed plane, features of the fingerprint image are extracted.
In addition, to remove the influence of noise against an image, a method for smoothing features of a ridge line has been disclosed in Japanese Patent Laid-Open Publication No. 5-181956 titled "Method for Smoothing Ridge Line Pattern and Apparatus Thereof" by Kamei and in "Classification of Fingerprint Patterns by Relaxation Method", the 22nd National Convention (1981 First Term Session), Information Processing Society of Japan by Kawakoshi et. al. The "Method for Smoothing Ridge Line Pattern and Apparatus Thereof" describes a technique based on the theory of minimizing energy. An evaluation function is assigned for an extracted direction for each two-dimensional local region of an image with a scale of reliability. By minimizing the evaluation function, the ridge line pattern is smoothed. On the other hand, in the "Classification of Fingerprint Patterns by Relaxation Method," information of directions extracted for each two-dimensional local region of an image is smoothed by so-called relaxation method.
In the technique described in Japanese Patent Examined Publication No. 5-746, since an image divided into local regions is processed, if a fingerprint/palmprint image locally deteriorates, the deteriorated image should be interpolated with information of adjacent local regions. However, in this related art reference, such an interpolating process is not performed. Thus, the fingerprint/palmprint image cannot be accurately obtained.
On the other hand, in the technique described in Japanese Patent Laid-Open Publication No. 5-181956, a sum of the sum E.sub.1 of differences between each local region of features of a ridge line extracted for each local region in the entire local regions and the weighted sum .alpha.E.sub.2 of the reliability of which the features of the ridge line are extracted from each local region is defined as an energy function. The weighted sum can expressed by Formula 1. The energy function is minimized so as to smooth the ridge line pattern. EQU E=E.sub.1 +.alpha.E.sub.2 (where .alpha.&lt;0) Formula 1!
However, if the above-described method is applied for wrinkles that frequently extend and go side by side at equal pitches in the wide range of a palm, the following problem will take place.
Such wrinkles have relatively good connectivity of features such as directions and pitches with adjacent regions, when components of wrinkles are selected, the entire energy may decrease. Where wrinkles are clearly present, the reliability for extracting features of ridge lines occasionally increases. Thus, when an image including such wrinkles is smoothed, the adjacent regions matched to the wrinkles thereof are also smoothed. Consequently, the wrinkles may be emphasized. In addition, it is difficult to define the energy function E that allows the above-described problem to be solved.
In the technique described in "Categorizing Fingerprint Patterns by Relaxation Method," the relaxation method is used as a technique for smoothing information with respect to directions extracted for individual local regions. Even in this technique, wrinkles that frequently extend and go side by side at equal pitches on a palmprint are smoothed such as matched to the wrinkle and emphasized.